How does it know it's coming from a mobile phone? Surely the webpage is the object posting the info to the payment gateway rather than the phone itself.
If we took an eCart created page and simplified it's design, but retained all of the eCart coding, then surely the payment gateway should still work... is that not right?
If I take a non-mobile ecommerce store, open it on my iPhone, so it is basically a miniaturised version of the desktop website, I am able to select products, add them to my basket, make a purchase and make payment... all from a bog standard website. Wouldn't a media query to change the css so the site fitted more comfortably on a phone screen (rather than be shrunk to fit) allow the code to function the same and enable payment to be made in the same way as that of a desktop browser?
Personally, I would expect an online store to work in the same way whether it is viewed in a desktop or mobile browser because it is the host server that is dealing with php and mysql code, the browser is there simply to read html and present it in a meaningful manner to humans isn't it?